1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
A typical wireless cellular network comprises many cells, with one or more base stations at each cell. A mobile user within a cell communicates with its serving base station of the cell. Since the locations of mobile users within a cell are random, the quality of the channel between a mobile user and its serving base station can vary significantly. For example, consider the two mobile users 120 and 122 in FIG. 1. Both mobile users 120 and 122 are in the same cell served by base station 112. Mobile user 120 is very close to base station 112, thus the quality of the channel between mobile user 120 and base station 112 can be fairly good. High data throughput can be achieved between mobile user 120 and base station 112.
On the other hand, mobile user 122 is at the edge of its serving cell and much further away from base station 112. In the downlink channel in which base station 112 sends signal to mobile user 122, the strength of the downlink signal that mobile user 122 receives attenuates more due to the increased distance. Further, since mobile user 122 is also closer to neighbor cells, it is also subjected to much stronger interferences of the signals from base stations 114 and 116 of neighbor cells. Consequently, the downlink channel quality can be very poor at the cell edge. In the uplink channel where mobile user 122 sends signal to base station 112, the uplink signal of mobile user 122 is also corrupted by other mobile users 124 and 126 in neighbor cells. Thus at the cell edge, the uplink channel quality can also be very poor. As a result, the data throughput at the cell edge can be much lower than the peak data rate achievable when a mobile user is in the very proximity of a base station. The low data throughput at the cell edge averages down the overall data throughput of the entire cell, thus significantly reducing the network performance.
Recently, in an effort to improve the network performance in terms of the data throughput, multipoint broadcasting schemes have been introduced to wireless cellular networks. Refer to FIG. 1, where mobile user 122 is connected to base station 112, mobile user 124 to base station 114, and mobile user 126 to base station 116. Without the multipoint broadcasting, each base station would communicate to its respective mobile user individually. Thus for mobile user 122, the signals it hears from base stations 114 and 116 appear as noises or interferences. If, as shown in FIG. 1, mobile user 122 is at the cell edge, then the interferences from base stations 114 and 116 can be much stronger than the signal from base station 112, and consequently mobile user 122 suffers much poorer communication quality, resulting in much lower data throughput.
A multipoint broadcasting scheme aims at increasing the cell-edge performance and can be described as follows. Refer to FIG. 1 where an example multipoint broadcasting system can be identified. Base stations 112, 114, and 116 form a set of collaborating multipoint broadcasters. Mobile users 122, 124, and 126 form a set of recipients in the multipoint broadcasting system. In the multipoint broadcasting system in FIG. 1, base stations 112, 114, and 116 transmit the combinations of the signals intended for mobile users 122, 124, and 126. For each base station, the combination “weight” for each mobile user signal can be different. Through elaborate algorithms, the signals are combined at each base station in such a way that when the transmitted signals from base stations 112, 114, and 116 arrive at mobile user 122, the signals for mobile users 124 and 126 are cancelled out or minimized, while the signal for mobile user 122 is maximized or enhanced, thus the signal quality of mobile user 122 improves significantly. Similarly, mobile users 124 and 126 will also see significant improvement in the quality of their respective signals. The combining of the signals at each base station is commonly referred to as “pre-coding”. The combining weights for each mobile-user signal and for each base station constitute the elements in a so called “pre-coding matrix”.
The signals from base stations in a cellular network are broadcast in nature. Thus a multipoint transmission scheme creates a set of multipoint broadcasting channels. With the ability to completely cancel the interference and to create clean channels for each mobile user, multipoint broadcasting channels are shown to have a capacity, a measure of the data throughput of the network, several times that of the traditional cellular networks. Multipoint broadcasting schemes have been adopted by advanced versions of LTE (long-term evolution, of the currently deployed third generation wireless cellular networks), under the name of “coordinated multipoint transmission”, or CoMP. The name follows from the fact that neighboring base stations coordinate to achieve multipoint broadcasting.
While multipoint broadcasting can bring tremendous benefits to wireless cellular networks, its performance depends critically on the availability of the downlink-channel information at the collaborating base stations. Consider the downlink multipoint broadcasting in FIG. 1. For the purpose of the interference cancellation, each of the base stations must have the channel information on all downlink channels between a base station and a mobile user. Generally, base stations are able to acquire only the uplink-channel information, and the downlink-channel information has to be acquired from the feedback by mobile users. The amount of the downlink-channel feedback data in LTE networks, however, can be extremely large for the following reasons. First, an LTE network utilizes very wide bandwidths with which the channel information grows proportionally. Second, wireless channel changes rapidly, which requires high feedback rate. Third, for applications such as multipoint broadcasting, there can be many downlink channels. For example, in the multipoint broadcasting system in FIG. 1, there are three base stations and three mobile users, thus there are nine downlink channels. Moreover, if base stations and/or mobile users have more than one antenna, as it has been increasingly the case, the number of downlink channels is further multiplied by the number of base station antennas and by the number of mobile user antennas. Fourth, for base stations to reliably recover the downlink-channel information, the feedback data has to be error-control coded, which further increases the effective feedback data size.
The huge downlink-channel feedback data results in extremely high feedback overhead. Since the downlink-channel feedback uses the uplink, the uplink capacity is reduced materially, and, in many cases, the feedback data leaves no room for the useful user data in the uplink.
There have been methods for reducing the feedback data size. One approach is to feed back only certain channel statistics, not the full channel information. This reduces the feedback overhead to an acceptable level at the expense of sacrificing most of the benefit of multipoint broadcasting. The improvement to the capacity of the cellular network is reduced from many folds to incremental. Such a high price is obviously unacceptable. Another approach is to use data compression techniques to compress the feedback data. This results in incremental reductions in the total feedback data but does not fundamentally change the fact that the feedback overhead is still unacceptably high.
Yet another approach is to use the time-domain representation of the channel for feedback. This is based on the assumption that the channel delay spread is short. The frequency-domain channel can be transformed into the time domain, and then truncated within the delay spread of the channel. While such a time-domain truncation approach may reduce the feedback overhead for short delay-spread channels, it falls short of its goal with channels with long delay spread. In fact, the time-domain channel samples spanned by the delay spreads of many typical wireless channels are often similar in size as the frequency-domain channel samples. This is particularly frequent at the cell edge where multipoint broadcasting is mostly desired and the efficient channel feedback is mostly needed.
The feedback overhead thus remains to be a major barrier to successful implementation of multipoint broadcasting in LTE networks. Since LTE has been rapidly adopted by the wireless industry as the future wireless cellular technology, the enormous feedback overhead is also a major barrier to multi-fold improvement of the wireless cellular network. Thus a strong need exists for a method, apparatus, and system that provide efficient downlink-channel feedback for LTE systems without compromising the capacity for useful data.